A very wide range of industrial processes use liquid droplets and solid particles of irregular shapes and sizes. Grinding powders, medical inhalers, and spray painting are just a few such examples. Industrial processes including coatings produced by thermal and other sprays typically involve determinations of particle parameters—for example, particle size, shape, velocity, and position in space. The area of aircraft icing involves supercooled water droplets in the presence of ice crystals and ice particles (spherical frozen droplets). Existing techniques cannot accurately and reliably measure the size of these particles. Furthermore, existing techniques cannot separate the liquid droplets from the ice particles and ice crystals.
Existing particle imaging techniques include incorporating bright-field imaging using arc flash lamps, pulsed lasers, and pulsed LEDs for illumination. These techniques typically use charge-coupled device (“CCD”) cameras or Complimentary Metal Oxide Semiconductor (“CMOS”) cameras to record the shadow images of the particles. These techniques typically use collimated or nearly collimated light with diffusers to illuminate the particle field. In these techniques, however, the out of focus particles under relatively dense particle field conditions typically produce shadows that complicate the detection and measurement of the in focus particle shadow images. In addition, larger particles in the light beam path can extinguish or obscure the light beam which causes a loss of the smaller particle image at the sample volume. Such losses of images result in an unacceptable bias in the sampling statistics.